1. Field of the Invention
The invention relates to a semiconductor switching device for switching at high frequencies, specifically to a compound semiconductor switching device operating at frequencies equal to or higher than 2.4 GHz.
2. Description of the Related Art
Mobile communication devices such as mobile telephones often utilize microwaves in the GHz range, and commonly need switching devices for high frequency signals which are used in switching circuits for changing antennas and switching circuits for transmitting and receiving such signals. A typical example of such a switching device can be found in Japanese Laid-Open Patent Application No. Hei 9-181642. Such a device often uses a field-effect transistor (called FET hereinafter) formed on a gallium arsenide (GaAs) substrate, as this material is suitable for use at high frequencies, and developments have been made in forming a monolithic microwave integrated circuit (MMIC) by integrating the aforementioned switching circuits.
FIG. 1A is a cross-sectional view of a conventional GaAs FET. The GaAs substrate 1 is initially without doping, and has beneath its surface an n-type channel region (or a channel layer) 2 formed by doping with n-type dopants. A gate electrode 3 is placed on the surface of the channel region 2, forming a Schottky contact, and a source electrode 4 and a drain electrode 5 are placed on both sides of the gate electrode 3, forming ohmic contacts to the surface of the channel region 2. In this transistor configuration, a voltage applied to the gate electrode 3 creates a depletion layer within the channel region 2 beneath the gate electrode 3 and, thus, controls the channel current between the source electrode 4 and the drain electrode 5.
FIG. 1B shows the basic circuit configuration of a conventional compound semiconductor switching device called a SPDT (Single Pole Double Throw) switch, using GaAs FETs. The source electrode (or the drain electrode) of each FET (FET1 and FET2) is connected to a common input terminal IN. The drain electrode (or source electrode) of each FET (FET1 and FET2) is connected to respective output terminals (OUT1 and OUT2). The gates of FET1 and FET2 are connected to the control terminals Ctl-1, Ctl-2 through resistors R1, R2, respectively. A pair of complementary signals is applied to the two control terminals, Ctl-1, Ctl-2. When a high level signal is applied to the control terminal of one of the FETs, the FET changes to an on-state, and a signal fed to the common input terminal IN passes through the FET and reaches one of the output terminals OUT1, OUT2. The role of the resistors R1 and R2 is to prevent leakage of the high frequency signals through the gate electrodes to the DC voltages applied to the control terminals Ctl-1, Ctl-2, which are substantially grounded at a high frequency.
The switching device shown in FIG. 1B must have shunts, which lead leaking signals to the ground, to attain a high degree of isolation. Alternatively, the gate width may be reduced to about 600 xcexcm without utilizing shunts so that the overall size of the switching device is reduced with a proper isolation, as described in commonly owned copending U.S. patent application Ser. No. 09/855,030, entitled xe2x80x9cCOMPOUND SEMICONDUCTOR DEVICE FOR SWITCHING.xe2x80x9d The disclosure of U.S. patent application Ser. No. 09/855,030 is, in its entirety, incorporated herein by reference.
The gate width may be further reduced to about 400 xcexcm while the switching device is still able to allow a linear input power (power hereinafter) as high as 20 dBm to pass through, which is required for applications such as BLUETOOTH and Wireless LAN, as described in another commonly owned copending U.S. patent application Ser. No. 10/105,802, entitled xe2x80x9cSEMICONDUCTOR SWITCHING DEVICE.xe2x80x9d The disclosure of U.S. patent application Ser. No. 10/105,802 is, in its entirety, incorporated herein by reference. This is achieved by using two FETs (FET1 and FET2) with different device characteristics including saturation current and pinch-off voltage.
However, the maximum power of 20 dBm may not be sufficient for other applications including ISM Band communications used in Wireless LAN with a high transmission rate.
The invention provides a semiconductor switching device including a first field effect transistor and a second field effect transistor. Each of the transistors includes a source electrode, a gate electrode and a drain electrode which are formed on the channel layer of the respective transistor. The first transistor has a larger gate width than the second transistor so that the first transistor has a higher saturation current than the second transistor. The switching device also includes a common input terminal connected to the source electrode or the drain electrode of the first transistor and connected to the source electrode or the drain electrode of the second transistor. A first output terminal is connected to the source electrode or the drain electrode of the first transistor which is not connected to the common input terminal. A second output terminal is connected to the source electrode or the drain electrode of the second transistor which is not connected to the common input terminal. In this configuration, the gates of the first transistor and the second transistor receive control signals so that one of the transistors opens as a switching element and the other of the transistors closes as another switching element. The gate width of the first transistor may be larger than 400 xcexcm while the gate width of the second transistor may be equal to or smaller than 400 xcexcm. Furthermore, the pinch-off voltage of the first transistor may be larger than the pinch-off voltage of the second transistor.